Dwelling furnaces of the type that recirculate dwelling air to a central furnace to be heated all operate on the same general principle. Dwelling air to be heated circulates around a closed system which is heated by burning a fuel, generally home heating oil, natural gas, propane, butane or mixtures thereof. Since burning the fuel results in the production of noxious combustion gases, it is vital that these gases not be introduced to the dwelling, but rather be exhausted through a chimney or flue to the atmosphere. The most important problem in modern furnace design is the extraction of the maximum amount of heat from the burned fuel before exhausting the combustion gases to the atmosphere.
Early designs consisted primarily of a heat exchanger having combustion chambers longitudinally arranged in relation to the flow of dwelling air to be heated such that fuel is introduced at a lower end where a flame causes heat to be generated. The heat rises through a series of internal baffles before exiting through an upper end of the combustion chamber into the chimney system. Concomitantly, circulated dwelling air passes around the outside of the heat exchangers to absorb heat through conduction, convection and radiation. Heat passes through the combustion chamber wall in a primary heat exchanger system. The system is relatively inefficient and combustion gas temperatures at the outlet of the furnace can exceed 500.degree. F.
In an attempt to capture more heat before the combustion gases are released to the atmosphere, more intricate baffling systems were implemented. However, the introduction of constricting baffles requires an induced draft blower to force the exhaust gases out of the furnace. An example of such design is disclosed in U.S. Pat. No. 4,261,326 to Ihlenfield, issued Apr. 14, 1981. Ihlenfield's design converted e.g., a three combustion chamber furnace into a two combustion chamber furnace having a secondary down-flow heat exchanger. Converting a combustion chamber (or primary heat exchanger) into a down-flow (or secondary heat exchanger) is effectuated by converting the air/fuel inlet of one primary heat exchanger into a exhaust outlet. At the same time, the two exhausts for the primary heat exchangers are placed in fluid communication with the now inlet end of the secondary heat exchanger. The induced draft blower placed beyond the secondary heat exchanger exhaust causes the combustion gases produced in the primary heat exchanger to be drawn through the top of the secondary heat exchanger downward through the secondary heat exchanger and out of the heat exchanger at the bottom (formerly the inlet) and exhausted by the induced draft blower into the flue system. Utilization of a secondary heat exchanger increases the efficiency to a claimed 85 to 87%. Nonetheless, exhaust temperatures for this type of furnace are in the vicinity of 200.degree. F. or higher.
Another innovation in high efficiency dwelling furnace design is the advent of the condensing or tertiary heat exchanger. This type of heat exchanger extracts heat from the combustion gases by effecting a phase change from the gaseous to liquid state. The condensation is accomplished by having the relatively cool dwelling air contact the surfaces of the tertiary heat exchanger before it is heated by either the primary or secondary heat exchanger. An example of a furnace utilizing the tertiary heat exchanger is found in Canadian Pat. No. 1,108,499, of Schaus, et al, issued Sept. 8, 1981. However, the Schaus, et al. furnace utilizes an unorthodox design comprising drum type heat exchangers having a central combustion chamber, and a secondary heat exchanger surrounding the combustion chamber which then fluidly connects to a vertically oriented tertiary condensation heat exchanger.